1. Field of the Invention
The present invention relates to a light amount control device for use in an optical apparatus, such as an electronic still camera, a video camera, or an interchangeable lens apparatus.
2. Related Background Art
An example of a stop device (a light amount control device) mounted in a conventional video camera is shown in FIG. 8.
This stop device is of a two-blade type, in which two aperture blades 123a and 123b are driven by a single rotary electromagnetic actuator (motor) 123c through a seesaw-type drive lever 123d. Symbol 123e indicates a base plate (i.e., casing) of the stop device.
Apart from this, there exists a stop device constructed as shown in FIGS. 9 and 10. FIG. 9 shows the state in which the aperture is fully open, and FIG. 10 shows the state in which the aperture is completely closed. FIG. 11 shows the construction of an optical system including the stop device.
In these drawings, symbol 113a indicates an actuator for opening and closing the aperture. The proximal end portion of a first lever (drive member) 113g is press-fitted onto an output shaft 113a1 of the actuator 113a. Pin portions 113c1 and 113c2 are formed at both ends of a second lever 113c. The pin portion 113c2 of the second lever 113c is engaged with a hole 113d1 of a first aperture blade 113d. On both sides with respect to the width direction of the first aperture blade 113d, there are formed elongated holes 113d2 and 113d3 extending in the vertical direction, and guide pins 113f1 and 113f2 formed on a base member 118 (see FIG. 11) of the stop device are engaged with the elongated holes 113d2 and 113d3.
The pin portion 113c1 of the second lever 113c is engaged with a hole 113e1 of a second aperture blade 113e. Formed on both sides with respect to the width direction of the second aperture blade 113e are vertically extending elongated holes 113e2 and 113e3, which are engaged with the guide pins 113f1 and 113f2. Further, the pin portion 113c1 is engaged with an elongated hole 113g1 formed in the first lever 113g. 
Further, for small-aperture correction, an ND filter 114 is attached to the first aperture blade 113d so as to cover an aperture of approximately F5.6 to F8.
As shown in FIG. 11, the stop device constructed as described above is used in an optical system having first through fourth lens units 101 through 104 arranged in order from the object side, and is arranged between the first and second lens units 101 and 102. Light transmitted through the aperture formed by the first and second aperture blades 113d and 113e effects image formation on a light receiving surface of an imaging device 106. Arranged between the fourth lens unit 104 and the imaging device 106 is an optical block 105 consisting of an infrared filter, a low-pass filter or the like.
In the above-described stop device, when the actuator 113a rotates, the second lever 113c rotates through the first lever 113g, with the result that the aperture blades 113d and 113e are driven vertically as seen in the drawing.
The conventional ND filter 114 used is one of fixed density or one of varying density. In the case of a fixed density filter, as the sensitivity of the imaging device increases, the density of the ND filter is increased to reduce the light transmission amount, the minimum aperture of the stop being increased if the brightness of the object is the same.
It should be noted, however, that, as shown in FIG. 11, when the density of the ND filter is thus increased, the difference in light amount between light “a” transmitted through the ND filter 114 and light “b” transmitted through the pass-through area of the aperture (the area with no ND filter 114) increases, resulting in a deterioration in resolution.
To overcome this problem, there has been proposed a stop device using an ND filter with a transmission characteristic in which, as shown in FIGS. 12 and 13, the light transmission amount (that is, density) varies at a substantially fixed rate.
However, whether the ND filter is of fixed density or variable density, immediately before the ND filter covers up an aperture of a desired F number, for example, of F5.6, a deviation in image forming position occurs between the light “a” transmitted through the ND filter 114 and the light “b” transmitted through the pass-through area due to the thickness and refractive index of the ND filter 114. Thus, in the intermediate stop area, in addition to the above-mentioned difference in light amount, a deviation in image forming position leads to a further deterioration in resolution.
Also in the case in which an ND filter of variable density is used, it is possible to mitigate the deterioration in resolution due to the above-mentioned difference in light amount; however, the pixel pitch of the imaging device is then as small as 3.2 to 3.8 μm, and in a lens unit in which the permissible circle-of-confusion diameter is small, such deviation in imaging position as mentioned above generates one-side unsharpness to an impermissible degree, resulting in a deterioration in image quality.
To obtain a satisfactory image formation performance even in the intermediate stop area, there has been proposed an ND filter which, as shown in FIG. 14, has, on the open side with respect to light attenuation portions 114a and 114a′ having a transmittance lower than 75%, an area (non-attenuation portion) 114b having a transmittance of 75% or more and formed of the same material and in the same thickness as those of these light attenuation portions. Moreover, as shown in the drawing, from the optical viewpoint, the non-attenuation portion 114b is set so as to be shifted from the open state to a position where it covers up the aperture, thereby making it possible to eliminate deviation in image forming position over the entire area of the variable range of the aperture.
However, as shown in FIG. 15, on the closed side, the ND filter 114 and the second lever 113c overlap and come into contact with each other in a shaded portion 114c (in particular, when there is warpage or the like in the ND filter 114), and the contact portion may make it impossible to perform smooth stopping operation.